The present invention relates to the art of magnetic resonance imaging. It finds particular application in conjunction with imaging body tissue in regions which contain moving tissues, organs, or the like and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also applicable to improving the clarity and freedom of artifacts in imaging sequences in which the desensitization or freezing of motion-related data is advantageous.
The human body undergoes at least four types of motion in areas which are commonly the subject of a magnetic resonance image. These motions include (1) blood flow, (2) respiratory and cardiac movement, (3) involuntary movement (e.g. the orbits of the eyes), and (4) peristalitic motion (e.g. arythmic motion of the esophagus and abdominal region). Commonly, a series of views, e.g. 256 views, is collected to form each image, with each view being displaced in time from the others. It was heretofore believed that motion-related artifacts were attributable to the inconsistency in the position of the moving tissue from view to view. Accordingly, the prior art collection techniques attempted to collect the views with the greatest possible position consistency from the view to view.
In one prior art technique, short TR/TE sequences were utilized with multiple averaging, i.e. each view is the average of more than one time displaced views collected with the moving tissue at different positions. This technique tended to average motion-related deviation among the views to create a pseudo-consistency from view to view. Another prior art technique utilized a multi-echo Carr-Purcell sequence in which data was collected at both first and second echoes. Differences between the first and second echo data could be attributed to blood flow and used to correct for flow related artifacts. In a third technique, cardiac or respiratory gating was utilized. That is, imaging sequences were permitted to occur only during specified portions of the respiratory and cardiac cycles. Commonly, imaging data collection was permitted only in the portions of the respiratory cycle with the least movement. For cardiac imaging, the imaging sequence was commonly synchronized with a selected phase of the cardiac cycle.
The use of short TR/TE sequences was disadvantageous in that it is limited to T1 weighted images. Further, multiple averaging increased scan times. Using the second echo of Carr-Purcell or other multi-echo sequences was only effective for rephasing material having a constant velocity. Breathing motion, surging blood flow adjacent to the heart, and the like do not have constant velocities and cannot be corrected with this technique. Further, the multi-echo technique requires a higher RF dosage than other T2 imaging techniques in order to desensitize the data to motion. With respiratory or cardiac gating, the time to collect image data is increased significantly and the image quality is degraded. Further, with cardiac gating, the rapid movement of the heart places limits on available pulse sequences--only short imaging sequences. Moreover, these prior art correction techniques were limited to correcting for blood flow and respiratory and cardiac motion. None of them were particularly effective for involuntary movement or peristalitic motion.
The present invention relates to a method and apparatus for imaging in which the image is desensitized to motion related artifacts by rephasing motion dependent components of the magnetic resonance signals.